This article attempts to establish a connection between the practical legitimation of war and the theories of international law, examining Sweden's efforts to justify her intervention in the Thirty Years War in 1630. Swedish argumentative strategy is analysed in the light of two major traditions of thinking about war: theological and humanist ‘just war’ traditions. The article argues that Swedish leaders did not appeal to the more belligerent humanist arguments which would have enabled them to describe their campaign as a just war either on the grounds of pre-emptive defence or humanitarian intervention. Instead, they tried to interpret it as being within the limits set by the more restrictive theological tradition. This strategy eventually forced them to relinquish attempts to present their intervention as a genuine war and to develop an argument of ‘police-action’, even though it resulted in a loss of credibility. The case study suggests that in the early seventeenth century the prevailing normative language of just war was that of the theologians.